Current equalizing circuit

ABSTRACT

A circuit for equalizing the AC input currents drawn by a polyphase rectifier. A plurality of sensing networks are connected in current sensing relationship to respective phases of a polyphase source, each sensing network providing an output voltage which is proportional to the current sensed thereby. The outputs of these sensing networks are connected in parallel to establish a plurality of circulating currents, the magnitude and direction of each circulating current being dependent upon the output voltage of all sensing networks. The latter currents are utilized to simultaneously change each of the sensed currents, as required, to achieve input current equality.

United States Patent Pohm [ 51 Mar. 21, 1972 [54] CURRENT EQUALIZINGCIRCUIT [72] Inventor: George H. Pohm, Lorain, Ohio [73] Assignee:Lorain Products Corporation 22 Filed: Apr. 21, 1971 211 App]. No.:135,982

[52] US. Cl.....-.....L ..321/5, 317/47, 320/60,

321/13, 321/19, 321/25, 321/27, 323/20 [51} Int. Cl. ..1'102m 7/00,H02h3/00, 605i [581 Field otSenrch ..321/5, 13, 19, 24, 25,27;

[56] References Cited UNITED STATES PATENTS 3,229,187 1/1966 Jensen..321/27 R 3,317,813 5/1967 Schaefer.... ..323/34 3,373,337 3/1968 Hung...323/24 X 3,401,308 9/1968 Darke ..317/47 POLYPHASE A- C SOURCE3,409,821 11/1968 Bingley ..323/24 X 3,566,245 2/1971 Blokker et a1...321/5 3,571,689 3/1971 Wise Primary Examiner-William H. Beha, Jr.Attorney-John Howard Smith [5 7] ABSTRACT A circuit for equalizing theAC input currents drawn by a polyphase rectifier. A plurality of sensingnetworks are connected in current sensing relationship to respectivephases of a polyphase source, each sensing network providing an outputvoltage which is proportional to the current sensed thereby.

' The outputs of these sensing networks are connected in parallel toestablish a plurality of circulating currents, the magnitude anddirection of each circulating current being dependent upon the outputvoltage of all sensing networks. The latter currents are utilized tosimultaneously change each of the sensed currents, as required, toachieve input current equality.

7 Claims, 2 Drawing Figures OUTPUT VOLTAGE SENSING 1711b FIRING NETWORKl7bc FIRING NETWORK PATENTEDMARZ] I972 I F e OUTPUT VOLTAGE SENSING l'37 POLYPHASE A- C SOURCE FlRING NETWORK TO A TOB

47Gb if FIG. 2

FIRING TO C NETWORK --o 1 l /Bbc TO LINE I CONDUCTORI l7cc| FIRING TOC"- TO A- NETWORK TO THYRISTOR l2ub TO THYRISTOR I30b INVENTOR.

GEORGE H. POHM ATTORNEY BACKGROUND OF THE INVENTION The presentinvention relates to control circuitry and is directed more particularlyto circuitry for equalizing the AC currents drawn by a polyphaserectifier.

In charging batteries which have high ampere-hour ratings, it has beenthe practice to utilize battery chargers that are energized withpolyphase AC input voltages. One reason is that a DC voltage produced byrectifying polyphase voltages requires less filtering than a DC voltageproduced by rectifying a single phase voltage. In such battery chargers,control of the magnitude of the output voltage is achieved by advancingor retarding the times at which various controlled rectifiers such asthyristors are rendered conducting. This is often accomplished byconnecting a feedback control circuit including a plurality of magneticamplifiers in switching control relationship to the controlledrectifiers.

Prior to the present invention, the problem has been that, due todifferences in the operating characteristics of the magnetic amplifiers,there occurred substantial differences in the average currents flowingthrough different controlled rectifiers. To assure that thesedifferences did not result in overheating, the current rating of eachcontrolled rectifier had V pearing therebetween. This interfered withthe operation of other polyphase loads.

SUMMARY OF THE INVENTION Accordingly, it is an object of the inventionto provide improved switching control circuitry.

Another object of the invention is to provide circuitry for balancingthecurrents which a polyphase circuit draws from a polyphase source.

Still another object of the invention is to provide circuitry foradvancing or retarding the firing of the controlled rectifiers of onephase of a polyphase circuit in accordance with the current in allphases of that circuit.

Yet another object of the invention is to provide circuitry responsiveto the current flowing in a plurality of polyphase line conductors forcontrolling a plurality of magnetic amplifiers which, in turn, controlthe currents flowing in all of those line conductors.

A further object of the invention is to provide circuitry wherein aplurality of magnetic amplifiers are controlled in accordance with thecirculating currents that flow between a plurality of sensing networks,each of which establishes an output voltage dependent upon the currentin a respective phase of a polyphase rectifier.

More specifically, it is an object ofthe invention to provide circuitryincluding a plurality of sensing networks for establishing voltageswhich are proportional to the currents in respective phases of apolyphase rectifier and circuitry for connecting the latter networks inmutually adjusting relationship through the control circuits of aplurality of magnetic amplifiers.

It is another object of the invention to provide circuitry for makingthe currents in different phases of a polyphase circuit approach acommon value and for holding those currents at that common value afterthe latter has been attained.

DESCRIPTION OF THE DRAWINGS FIG. I is a combined schematic and blockdiagram showing one embodiment of the circuit of theinvention and FIG. 2is a schematic diagram showing the circuitry appear ing in block form inFIG. 1.

DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown, forthe purpose of explanation, an AC to DC converter for charging a battery10 from a three-phase AC source 11. In the present embodiment theconverter circuit includes a polyphase transformer having threeprimary-secondary winding pairs AB and ab, BC and bc and CA and'ca.These winding pairs may be located on respective legs of a single,three-legged core, as indicated by the dotted lines in FIG. 1, or may belocated on separate cores. The converter circuit also includes aplurality of pairs of alternately conducting switching means l2ab andl3ab, l2bc and l3 bc and l2ca and 13m which here take the form ofthyristors.

Primary windings AB, BC and CA may be connected in a delta configurationbetween polyphase line conductors A, B and C. Secondary windings ab, beand ca are center-tapped and connected to battery 10 through a positivebus P and a negative bus N. Secondary winding ab, for example, isconnected to positive bus P at its center tap and to negative bus N atits end through thyristors Hub and l3ab and an inter-phase transformer14. The latter serves to lower the peak current flowing through eachthyristor while increasing the conduction period thereof.

When the voltage across primary winding AB has a first polarity,conduction is initiated in thyristor l2ab to establish a downwardflowing charging current through battery 10. Similarly, after thevoltage across primary winding AB reverses, conduction is initiated inthyristor l3ab to again establish a downward flowing charging currentthrough battery 10. Since battery 10 charges during both half-cycles ofthe AC voltage across primary winding AB, it will be seen that secondarywinding ab and thyristors 12ab and l3ab are connected as a full waverectifying network between primary winding AB and battery 10.

The average value of the above described charging current depends uponthe time during each half-cycle when thyristor conduction is initiated.If, for example, conduction begins late in the half-cycle, the averagevalue of the charging current through battery 10 (and, therefore, themagnitude of the AC input current drawn from source 11 by primary AB)will be relatively small. If, on the other hand, conduction begins earlyin the half-cycle, the average value of the charging current throughbattery 10 (and, therefore, the'magnitude of the AC input current drawnfrom source 11 by primary AB) will be relatively larger.

It will be understood that the above remarks are equally applicable tothe rectifying networks including secondary windings be and ca andthyristor pairs 12bc and 13bc and 12m and a, respectively. The latternetworks are connected to battery 10, in power-aiding relationship tothe first described rectifying network, through conductors l5 and 16.

To the end that the conduction of the above described rectifyingnetworks may be controlled to afford the desired load voltage regulationand input current equalization, there are provided firing networks l7ab,17bc and l7ca. As shown in FIG. 2, firing network 17ab includes firingpulse generating means which here takes the form of a line voltageclipping network l8ab and controllable conducting means which here takesthe form of a magnetic amplifier l9ab.

As will be described more fully presently, clipping network l8abutilizes the voltage between line conductors A and B to generatesubstantially square wave voltages on conductor pairs 20ab and 2lab.These voltages are applied to the gatecathode circuits of thyristors Huband 13ab,-respectively, through the gate windings 22ab and 23ab,respectively, of magnetic amplifier l9ab. These gate windings allow thetimes at which gate-cathode control currents begin to flow in thyristors12ab and l3ab to be controlled in accordance with the currents flowingin the control windings 2411b, 25ab and 26ab and, thereby, in accordancewith the DC output voltage and AC input current. It will be understoodthat firing networks 17120 and 17m include similar networks and functionin a similar manner.

To the end that pulse generating network l8ab may provide the desiredthyristor firing pulses, the latter network includes a full wave bridgerectifier comprising'diodes 2711b, 28ab, 29ab and 30ab, a couplingtransformer 31ab having a primary winding 3211b and first and secondsecondary windings 33ab and 34ab, and a breakdown or zener diode 35ab.-When line conductor A is positive from line conductor B by a voltagesufficient to breakdown diode 35ab, current flows from conductor A toconductor B through diode 27ab, zener diode 35ab, the base-emittercircuit of a transistor 36ab, diode 30ab and a current limiting resistorR. Under these conditions, the voltage across primary winding 3211b hasa first polarity and is equal to the sum of the voltage drops across theabove named circuit elements, each of which is substantially constantand independent of the magnitude of current flow. Transistor 36ab servesto keep the current through zener diode 35ab at a low level by shuntingcurrent therearound.

Similarly, when the polarity of the voltage between line conductors Aand B reverses, current flows through zener diode 35ab and transistor3611b through diodes 2811b and 2911b. Under the latter conditions, thevoltage across primary winding 3211!), has a second, reversed polarityand a magnitude which is substantially constant and equal to thatdescribed above. Thus, a substantially squarewave of voltage appearsacross primary winding 32ab and secondary windings 3311b and 3411b.

In order that the above squarewave voltages may be utilized to controlthe conduction of thyristors l2ab and 1311b in accordance with themagnitude of the DC output voltageand the imbalance between thepolyphase input current, control winding 2511b of magnetic amplifierl9ab is connected to a suitable output voltage sensing network 37 andcontrol winding 2411b thereof is connected to current sensing means38ab. Additional control windings include a bias winding 26ab which isconnected to a suitable bias source 39ab and a short circuited controlwinding 40ab which assists in resetting the flux in magnetic amplifiercores 4lab and 42ab.

In the present embodiment, input current sensing network 3811b includesa current transformer 43ab having a one turn primary through which flowsthe current in primary winding AB and a secondary winding 4411b wound ona toroidal core 45ab. Sensing network 3811b also includes a full waverectifying bridge 4611b and a filtering network include a capacitor 47aband a bleeder resistor 48ab. This network serves to provide a DC voltageat output terminals 38ab, and 38ab which is a function of the AC currentflowing in primary winding AB. It will be understood that sensingnetworks 3811c and 380a operate in a similar manner to provide DC outputvoltages which are functions of the AC currents in primary windings BCand CA.

In order that sensing networks 386b, 3812c and 38ca may control firingnetworks l7ab, l7bc and 17m to assure the desired substantially equal ACinput current distribution, positive sensing network output terminals3812b 38120 and 38m are connected together by a conductor 49 and thenegative sensing network output terminals 38ab 38110 and 38011; areconnected together by a conductor 50 through control windings 24ab, 24bcand 240a. This assures that the current in each of the above controlwindings is influenced by the AC input currents measured by the othersensing networks.

Since the outputs of sensing networks 38ab, 38bc and 38m are connectedin parallel by conductors 49 and 50 and control windings 2411b, 24bc and24m, the voltage between the latter conductors is a function of thevoltages produced by all sensing networks. The magnitude of this voltageis dependent upon the output voltage of each sensing network and uponthe magnitudes and directions of the currents in control windings 24ab,24bc and 240a, these currents being, in turn, dependent upon differencesin the output voltages of sensing networks 38ab', "3811c and 38m.

Assuming, for example, that the current in primary winding CA is greaterthan the current in either primary winding AB or primary winding BC, theoutput voltage of sensing network 380a will be greater than the outputvoltages of sensing networks 38ab and 38bc. Because these outputs areconnected in parallel by conductors 49 and 50 through control windings2441b, 24hr: and 24m, it is apparent that circulating currents will flowamong the latter. These circulating currents will distribute themselvesso that the voltage across each series branch including a controlwinding and a sensing network output will be equal to the voltage acrosseach other series branch. The resulting voltage appears betweenconductors 49 and 50.

In the present example, current will flow out of sensing network output380a to produce a voltage drops across control winding 240a which isequal to the difference between the output voltage of network 38m andthe voltage between conductors 49 and 50. At the same time, current willflow into sensing network outputs 38ab and 38bc, to produce voltagedrops across control windings 24ab and 24170 which are equal to thedifferences between the output voltages of networks 38ab and 38bc andthe voltage between conductors 49 and 50. Thus, the magnitudes anddirections of current flow through control windings 24ab, 2417c and 240aare dependent upon differences between the output voltages of sensingnetworks 38ab, 38hr and 380a and, therefore, dependent upon differencesbetween the currents in primary windings AB, BC and CA.

To accomplish the objective of the invention, the above describedcirculating currents are utilized to eliminate any inequality in the ACinput currents flowing in AC line conductors A, B and C. To the end thatthis may be accomplished,

each control winding is arranged to delay the conduction of therespective gate winding if current flows out of the positive sensingnetwork output in series therewith, and to advance the conduction ofthat switching if current flows into the positive sensing network outputin series therewith. This assures that those sensing networks which havea high output voltage, due

to their association with primary windings having excessive currents,produce outward flowing current and thereby reduce the currents in therespective primary windings and that those sensing networks which havelower output voltages, due to their association with primary windingshaving insufficient currents, produce inward flowing currents andthereby increase the currents in the respective primary windings. Thus,the magnitudes of the primary currents are made to approach a value atwhich they will be equal to one another. It will be understood that oncea substantially equal input current distribution is achieved, the abovedescribed circulating currents will maintain the condition of equality.

As shown in FIG. 2, control winding 24ab is wound so that the flow of acurrent in a counter-clockwise direction therethrough (the direction offlow of current flowing out of positive sensing network output 38abopposes the flux produced by gate windings 22ab and 23ab, and so thatthe flow of a current in a clockwise direction through control winding24ab (the direction of flow of current flowing into positive sensingnetwork output 38ab aids the flux produced by gate windings 22ab and23ab. Consequently, it will be seen that the flow of current intosensing network input 38ab delays the firing of thyristor 12ab and 13aband thereby reduces the magnitude of the AC current in primary windingAB and that the flow of current out of sensing network input 38abhastens the firing of thyristors l2ab and l3ab and thereby increases themagnitude of the AC current in primary winding AB. Thus, extreme valuesof primary current are eliminated by the flow of circulating currentsbetween parallel connected control windings.

Because the above described circulating currents flow in response todifferences in the output voltages of the sensing networks and,therefore, in response to differences in the currents in primary windingAB, BC and CA, it will be seen that the circuit of the inventionoperates to establish primary current equality. As this equality isapproached, the capacitor voltages will approach equality and thereby,reduce the flow of the above circulating currents. Thus, controlwindings 24ab, 24bc and 2401: will not interfere with other controlsover firing networks l7ab, l7bc and 170a once input current equality isachieved.

From the foregoing, it will be seen that a switching control circuitconstructed in accordance with the invention is adapted to sense themagnitudes of a plurality of currents and to utilize the differencestherebetween to equalize the magnitudes thereof, this being accomplishedby utilizing a novel circuit arrangement responsive to differences inthat plurality of currents to control each such current.

It will be understood that the above embodiment is for explanatorypurposes only and may be changed or modified without departing from thespirit and scope of the appended claims.

. What is claimed is:

1. In a current control circuit for equalizing the currents which flowin different phases of a polyphase network, in com-' bination, polyphaseAC terminal means, DC terminal means, 1

switching means for controlling the flow of current between said AC andDC terminal means, means for connecting said switching means in currentcontrol relationship between said AC and DC terminal means, a pluralityof current sensing networks each including first and second outputterminals, said sensing networks serving to establish output voltageswhich are substantially proportional to the currents sensed thereby,means for connecting said sensing networks in current sensingrelationship to respective phases of said polyphase network, a pluralityof firing networks each including at least first and second inputterminals, means for connecting said firing networks in conductioncontrol relationship to predetermined switching means, means forconnecting together said first output terminals of said sensingnetworks, means for connecting together said first input terminals ofsaid firing networksand means for connecting the second output terminalof each sensing network to the second input terminal of a respectivefiring network.

2. In a current control circuit for equalizing the currents which flowin different phases of a polyphase network, in combination, polyphase ACterminal means, DC terminal means, switching means for controllingtheflow of current between said AC and DC terminal means, means forconnecting said switching means in current conducting relationshipbetween said AC and DC terminal means, a plurality of current sensingnetworks each including output means, means for connecting said sensingnetwork in current sensing relationship to respective phases of saidpolyphase source, each sensing network serving to establish an outputvoltage which varies in accordance with the current sensed thereby, aplurality of firing networks each including input means whereby thefiring activity thereof may be advanced or retarded and output means,means for connecting the output means of said firing networks inconduction control relationship to respective switching means, means forconnecting the output means of each of said sensing networks in serieswith the input means of the respeclive one of said firing networks toestablish a plurality of series branches and means for connecting saidseries branches in parallel, circulating current establishingrelationship to one another to achieve interdependent control of saidfiring networks.

3. A current equalizing circuit as set forth in claim 2 wherein saidcurrent sensing networks each include a current transformer, an AC andDC converter network and means for connecting the former to the latter.

4. A current equalizing circuit as set forth in claim 2 wherein each ofsaid firing networks includes a magnetic amplifier including gatewinding means and control winding means, means for connecting said gatewinding means to the which flow in different Ehases of a polyphasenetwork, in combination, polyphase A terminal means, a polyphasetransformer having a plurality of primary and secondary windings, meansfor connecting said polyphase AC terminal means to said primarywindings, a load, switching means for controlling the flow of currentbetween said load and said secondary windings, means for connecting saidswitching means between said secondary, windings and said load, aplurality of current sensing networks each having AC input means and DCoutput means, means for connecting the input means of said sensingnetworks in current sensing relationship to respective primary windings,a plurality of firing networks each having input means and output means,means for connecting the output means of said firing networks inconduction control relationship to respective switching means and meansfor connecting the outputs of said sensing networks to the input meansof said firing networks to afford simultaneous, interdependent controlthereof.

6. In a current control circuit for equalizing the currents which flowin different phases of a polyphase network, in combination, polyphase ACtenninal means, DC terminal means, a polyphase switching circuit forcontrolling the flow of current between said AC terminal means and saidterminal means, said switching circuit including a plurality ofswitching means for controlling the magnitude of current in respectivephases of said polyphase network, a plurality of sensing networks eachhaving output means, means for connecting said sensing networks incurrent sensing relationship to each phase of said polyphase network, aplurality of means for firing said switching'means, said firing meansbeing adapted to hasten or delay the firing of said switching means inaccordance with the magnitude and direction of current at the inputsthereof, and means for controlling the magnitudes and directions ofcurrents at the inputs of said firing means in accordance withdifferences in the voltages at the outputs of said sensing networks.

7. In a current control circuit for equalizing the currents which flowin different phases of a polyphase network, in combination, polyphase ACterminal means, a transformer having a plurality of primary windings anda plurality of secondary windings, means for connecting said AC terminalmeans to said primary windings, a plurality of controlled rectifyingnetworks, each of said rectifying networks including a plurality ofswitching control inputs, a DC load, means for connecting saidrectifying networks between said secondary windings and said DC load,firing pulse generating means, means for connecting said source inenergizing relationship to said pulse generating means, variableconducting means having input means and output means, means forconnecting the output means of said variable conducting means betweenrespective pulse generating means and the switching control inputs ofrespective rectifying networks, current sensing means, means for passingsaid primary currents through said current sensing means, means forconnecting said sensing means to the inputs of respective variableconducting means and means for connecting together the inputs of saidvariable conducting means, each sensing means serving to increase theconductivity of the respective rectifying network when current flowstherethrough in one direction and to decrease the conductivity of therespective rectifying network when current flows therethrough in theother direction.

' STATES PATENT OFFICE v CERTIFICATE 0E CORRECTION Pat ent No 3,651, 393I I Dated M h 2] 1222 I 3 Inventor-(s) George -H. Pohm- It is certifiedthat error appear s in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Claim 3, line-3, change "and", first occurrence, to, --t ov- Claim 6,line 5, after "said.",. second occurrence, insert Signed and sealed thisISthIday of August 1972.

(SEAL) Attest:

EDWARD M.FLETGHER ,JR. v ROBERT GOTTSCHALK v Commissioner of Patents Attesting Officer FORM USCOMM-DC 60376-P69 I ".5. GOVERNMENT PRINTING OFFlCF-Z I989 S6-l3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent No. 3,651, 393 Dated mgr b 2] |2Z2 Inventor(s) George -H. Pohm Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Claim 3, line 3, change "and", first occurrence, to --to--.

Claim 6, line 5, after "said", second. occurrence, insert Signed andsealed this'lSthIday of August v1-972.

(SEAL) Attest:

EDWARD M.FLETGHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents USCOMM-DC 60376-P69 u,s. sovenuusm PRINTING omce: I959oaee-au FORM FO-105O (10-69)

1. In a current control circuit for equalizing the currents which flowin different phases of a polyphase network, in combination, polyphase ACterminal means, DC terminal means, switching means for controlling theflow of current between said AC and DC terminal means, means forconnecting said switching means in current control relatioNship betweensaid AC and DC terminal means, a plurality of current sensing networkseach including first and second output terminals, said sensing networksserving to establish output voltages which are substantiallyproportional to the currents sensed thereby, means for connecting saidsensing networks in current sensing relationship to respective phases ofsaid polyphase network, a plurality of firing networks each including atleast first and second input terminals, means for connecting said firingnetworks in conduction control relationship to predetermined switchingmeans, means for connecting together said first output terminals of saidsensing networks, means for connecting together said first inputterminals of said firing networks and means for connecting the secondoutput terminal of each sensing network to the second input terminal ofa respective firing network.
 2. In a current control circuit forequalizing the currents which flow in different phases of a polyphasenetwork, in combination, polyphase AC terminal means, DC terminal means,switching means for controlling the flow of current between said AC andDC terminal means, means for connecting said switching means in currentconducting relationship between said AC and DC terminal means, aplurality of current sensing networks each including output means, meansfor connecting said sensing network in current sensing relationship torespective phases of said polyphase source, each sensing network servingto establish an output voltage which varies in accordance with thecurrent sensed thereby, a plurality of firing networks each includinginput means whereby the firing activity thereof may be advanced orretarded and output means, means for connecting the output means of saidfiring networks in conduction control relationship to respectiveswitching means, means for connecting the output means of each of saidsensing networks in series with the input means of the respective one ofsaid firing networks to establish a plurality of series branches andmeans for connecting said series branches in parallel, circulatingcurrent establishing relationship to one another to achieveinterdependent control of said firing networks.
 3. A current equalizingcircuit as set forth in claim 2 wherein said current sensing networkseach include a current transformer, an AC and DC converter network andmeans for connecting the former to the latter.
 4. A current equalizingcircuit as set forth in claim 2 wherein each of said firing networksincludes a magnetic amplifier including gate winding means and controlwinding means, means for connecting said gate winding means to theoutput means of said firing network, means for connecting at least oneof said control winding means to the input means of said firing network.5. In a current control circuit for equalizing the currents which flowin different phases of a polyphase network, in combination, polyphase ACterminal means, a polyphase transformer having a plurality of primaryand secondary windings, means for connecting said polyphase AC terminalmeans to said primary windings, a load, switching means for controllingthe flow of current between said load and said secondary windings, meansfor connecting said switching means between said secondary windings andsaid load, a plurality of current sensing networks each having AC inputmeans and DC output means, means for connecting the input means of saidsensing networks in current sensing relationship to respective primarywindings, a plurality of firing networks each having input means andoutput means, means for connecting the output means of said firingnetworks in conduction control relationship to respective switchingmeans and means for connecting the outputs of said sensing networks tothe input means of said firing networks to afford simultaneous,interdependent control thereof.
 6. In a current control circuit forequalizing the currents which flow in different phases of a polyphasenetwork, in combination, polyphase AC terminal means, DC terminal means,a polyphase switching circuit for controlling the flow of currentbetween said AC terminal means and said terminal means, said switchingcircuit including a plurality of switching means for controlling themagnitude of current in respective phases of said polyphase network, aplurality of sensing networks each having output means, means forconnecting said sensing networks in current sensing relationship to eachphase of said polyphase network, a plurality of means for firing saidswitching means, said firing means being adapted to hasten or delay thefiring of said switching means in accordance with the magnitude anddirection of current at the inputs thereof, and means for controllingthe magnitudes and directions of currents at the inputs of said firingmeans in accordance with differences in the voltages at the outputs ofsaid sensing networks.
 7. In a current control circuit for equalizingthe currents which flow in different phases of a polyphase network, incombination, polyphase AC terminal means, a transformer having aplurality of primary windings and a plurality of secondary windings,means for connecting said AC terminal means to said primary windings, aplurality of controlled rectifying networks, each of said rectifyingnetworks including a plurality of switching control inputs, a DC load,means for connecting said rectifying networks between said secondarywindings and said DC load, firing pulse generating means, means forconnecting said source in energizing relationship to said pulsegenerating means, variable conducting means having input means andoutput means, means for connecting the output means of said variableconducting means between respective pulse generating means and theswitching control inputs of respective rectifying networks, currentsensing means, means for passing said primary currents through saidcurrent sensing means, means for connecting said sensing means to theinputs of respective variable conducting means and means for connectingtogether the inputs of said variable conducting means, each sensingmeans serving to increase the conductivity of the respective rectifyingnetwork when current flows therethrough in one direction and to decreasethe conductivity of the respective rectifying network when current flowstherethrough in the other direction.